Nothing would stop us from logging to the same application username from multiple clients at the same time. However, when it comes to the license we can share the same account between many users and consider it as one concurrent user. For license question please see the link below
http://www.oracle.com/us/corporate/pricing/index.html
To get Core Processor Licensing Factor,
http://www.oracle.com/us/corporate/contracts/processor-core-factor-table-070634.pdf
